An energy harvester is a device that converts mechanical movements into electrical energy. This electrical energy can then be stored or used by other devices. Thus, an energy harvester of this type can produce useful electrical power from vibrations. For example, the vibrations of an air duct can be converted to electrical energy by an energy harvester and the electrical energy can then be used to power a sensor that measures the temperature of air in that duct. Therefore, the sensor will not require electrical wiring to a remote source of power or periodic battery changes.
Applications of such energy harvesters include supplementing other power sources or recharging batteries, thereby extending battery life; elimination of wiring for electrical devices remote from a power source, the powering of mobile electronic instruments as well as powering wireless monitoring and communications devices. These latter applications typically comprise the sensing of local conditions to generate monitoring data, optionally, the processing of that data and the wireless communication of the data to a central data processing point. Such applications include wireless health monitoring; wireless monitoring of temperature, air flow, humidity, and gas content in heating, ventilation and air-conditioning (HVAC) systems; wireless monitoring of traffic flow, turbulence, noise, troop or other personnel movements; wireless, self-powered security systems; and “condition-based maintenance” systems including passive detection of creep or crack propagation in structures.
There are a variety of conventional devices for generating electrical power from vibrations, oscillations or other mechanical motions. These devices include inductive devices, capacitive devices, and piezoelectric devices. Inductive devices that convert vibrations to electrical power essentially work like an acoustic speaker (in which electrical signals are converted into vibrations of the speaker cone) in reverse. This operation can also be considered on the basis of the generator principle, that is, Faraday's law of induction:       V    ⁡          (      t      )        =      N    ⁢                  ∂        B                    ∂        t              ⁢    A  
The voltage generated by induction is proportional to the number of turns, N, in an electrical winding and the rate of flux change through those windings                     ∂        B                    ∂        t              ⁢    A    ,where ∂B is the flux density change during the vibration and A is the area of the coil through which the flux change perpendicular to the coil plane is measured by the N turns.
One problem with conventional inductive energy harvesters is that the voltage generated decreases for vibrations at lower frequencies. Also, their energy density is reported to be low. In order to increase output voltage, either the product NA must be increased or the flux change ∂B must be increased. Consequently, the power produced by inductive energy harvesters is presently limited by coil size (to increase NA), the need for heavy, powerful permanent magnets to produce a large flux density change ∂B, and large vibration frequencies (to increase       (          to      ⁢                          ⁢      increase      ⁢                        ∂          B                          ∂          t                      )    .Typical reported output voltages are low unless the device is large. For example, with a flux ∂B=0.5 Tesla coupled to a 10 Hz vibration so that             ∂      B              ∂      t        =            2      ⁢      π      ⁢                          ⁢      f      ⁢                          ⁢      Δ      ⁢                          ⁢      B        ≈    100  Tesla/second, a device with a one cm2 area sensed by a 500 turn coil generates an induced voltage of approximately five volts into an infinite load impedance. However, in a practical system, as the load impedance decreases, current flows and, in accordance with Lenz's law, generates a back EMF that opposes the motion of the magnet and opposes the induced voltage thereby reducing the power output. Consequently, small systems described in the literature report an average power output of approximately only 0.3 microwatts (for example, see “Development of an Electromagnetic Microgenerator”, C. Shearwood and R. B. Yates, Electronics Letters, v. 13, p. 1883 (1997)). The maximum power output of another small inductive energy harvester has been estimated to be 400 microwatts (“Self-Powered Signal Processing Using Vibration-Based Power Generation”, R. Amirtharaja and A. Chandarakasan, IEEE Journal of Solid State Circuits, v. 33, n. 5, pp. 687–695 (1998). The size of these devices indicates that the power density that can be achieved by inductive harvesters is in the range of 0.005 to 0.5 milliwatts/cm3.
Capacitive devices make use of the capacitor equation:       Q    ⁡          (      t      )        =      CV    =                  κɛ        0            ⁢                        A          ⁡                      (            t            )                                    d          ⁡                      (            t            )                              ⁢      V      The devices are arranged so that external vibrations vary the capacitor plate overlap area (A) and/or the capacitor plate spacing (d). Thus, a vibration causes a change in charge on the capacitor when a voltage is applied to the device. When the capacitor is used to drive a load, the charge flow is damped with a characteristic decay time given by the time constant, τ=RC. Both the inductive and capacitive devices generate an electrical signal that varies with a frequency that is the same as the vibration frequency.
Capacitive vibration energy harvesters have several drawbacks. First, they require an auxiliary source of power, such as a battery, and use some of the generated power to run the device. Secondly, as with the inductive devices described above, the power generated by capacitive devices and the resulting energy density is relatively modest. Literature reports for micro-electro-mechanical (MEMS) inter-digitated capacitor energy harvesters indicate an average power of eight microwatts. See “Vibration-to-electric energy conversion” S. Meninger, J. O. Mur-Miranda, R. Amirtharajah, A. Chandrakasan, et al., IEEE Transactions on VLSI Systems, v. 9, n. 1, p. 64 (2001). The very small size of this MEMS device puts its energy density in the range of 0.01 to 0.1 milliwatts/cm3.
Piezoelectric materials generate a voltage when they are stressed in accordance with a piezoelectric equation:   v  =            g      33      piezo        ⁢          σ      ·      d      where gij is a coefficient that describes the ability of the piezoelectric material to convert a stress to a voltage, σ is the stress applied to the piezoelectric material, and d is the spacing between electrodes that measure the voltage. Ceramic (polycrystalline) piezoelectric elements, flexible piezoelectric fiber composites, or polymeric electroactive materials can be used in various energy harvesting applications. One proposed class of electroactive energy harvesters makes use of the periodic compression in the heel of a shoe or boot caused by walking to stress a piezoelectric material in order to generate power. Walking generates a stress on the order of 200 lbs. over 10 in2 or 1 to 2×105 Pa. With piezoelectric stress coupling coefficients typically in the range 5-20 millivolt/(meter-Pa), the voltage generated by a piezoelectric energy harvester in such an application would be of order 1.3 volts, with a power density (½CV2) on the order of 2/R watts/cm3.
In a real device constructed with piezoelectric polymers, walking impact generated an average power of approximately 8 milliwatts (“Energy Scavenging with Shoe-Mounted Piezoelectrics”, N. S. Shenck and J. A. Paradiso, IEEE Microelectronics, v. 21, n. 3, May-June 2001, p. 30–42) Another device using piezoelectric fiber composites projects an ultimate average power density of approximately 0.1 milliwatts/cm3 (“Compact Piezoelectric Based Power generation”, K. Ghandi, Continuum Controls, Inc., DARPA Energy Harvesting Program Review, 2000). The small size of these devices puts their energy densities in the range of 0.1 to 1.0 milliwatts/cm3 with projections up to 5 millwatts/cm3.
Therefore, there is a need for an energy harvesting device with improved output power.